How to candle blue eggs?

I have a broody Cream Legbar who has been sitting for a while. She started with 4 eggs, but since she was in a nest box for a while, she has now 7.
It's my first time, for her too. At first I thought she was still laying, then I saw it was other hens laying in the box while she was sitting.
Given the uncertain starting time of incubation, I have tried to candle the eggs, but no luck, the shell is too thick and hard.
One sounds like there is something hard in it floating in fluid ?! Nothing with the others.
Any tips for candling such eggs? Also I'm worried I may have harmed the possible chick in the "floating/bumpy" egg when I shook it.
It has been 3 weeks since she started with her 4 original eggs. How long can I let her sit before calling it quit?
Thanks .
Worried first timer.

I don't know what I was thinking. It was the last one and by then I guess I had concluded that none were fertile.
As I mentioned I have not been able to see through the shell.
I use a very powerful flashlight in a dark room.

At 3 weeks the egg should be almost completely dark when you candle it, except for a large air cell when you put the light at the top (big end) of the egg; the chick would be taking up nearly all the available space. I usually find blue eggs relatively easy to candle but as lazy gardener said, you do need a completely dark room. I would recommend Brinsea’s high intensity egg candler for those really thick or dark eggs. I started off using a flashlight but unless you get a really good seal between egg and light source too much light escapes making it difficult to see. If the whole egg lights up like a light bulb at 3 weeks it’s definitely not fertile. I hope this helps

Always a good idea to check fertility on the eggs (if using your own) before putting any eggs in the incubator. For those dark eggs, I use a table lamp with 7W CFL, and make a candling tube out of a shampoo bottle and some tinfoil to make the tube completely light proof.

Thank you Joeschooks. I will give candling another try. I found a neoprene sleeve I can wrap on the flashlight. I think it will work. Wish I had thought about that sooner. I even didn't think she was serious about it at first. We'll see.